Balor

Balor only had one eye which was usually closed; its gaze killed anyone that saw it.

According to a prophecy, Balor was to be killed by his grandson. He locked his daughter, Ethlinn, in a tower made of crystal to keep her from becoming pregnant. However, Cian, one of the Tuatha de Danaan, with the help of the druidess Birog, managed to enter the tower. She gave birth to a son by him, but Balor threw him into the ocean. Birog saved him and gave him to Manannan mac Lir, who became his foster father. He was called Lugh Lamhfada and became a member of the Tuatha de Danaan.

Lugh Lamhfada led the Tuatha in the Second Battle of Mag Tuireadh[?] against the Fomorians. Ogma disarmed Balor during this battle and Balor killed King Nuada with his eye, but Lugh ripped the fatal eye out with a sling, killing Balor.